| dc.description.abstract | In order to address the issues of thermal loss, transmission distance, and cost in
the current technologies by Complementary Metal Oxide Semiconductor (CMOS)
processes, silicon photonics technology has been gradually developing. It features low
transmission loss, high bandwidth, and good compatibility with CMOS processes.
Silicon photonics integrated optoelectronic circuits have great potential in applications
such as optical communication, biosensing, quantum optics, and nonlinear optics. The
commonly used wavelength range for silicon photonics is 1310 nm and 1550 nm, as
silicon-based materials exhibit less absorption in these ranges, resulting in lower losses
for optical waveguides. However, due to the small bandgap of silicon waveguides, they
are prone to two-photon absorption, limiting their applications in nonlinear optics.
Therefore, research efforts have focused on developing alternative materials as
waveguide media. This study utilizes gallium nitride (GaN) and silicon nitride (SiNx)
as waveguide core materials. GaN possesses asymmetric crystal lattice arrangement,
allowing for both χ(2) and χ(3) nonlinearities, making it more suitable for second
harmonic generation and frequency comb generation. Traditionally, GaN is deposited
using Metal Organic Chemical Vapor Deposition (MOCVD). But due to the lattice
mismatch between GaN and silicon-based materials, MOCVD cannot be used for
deposition of high-quality thin film on silicon or silicon oxide substrates. Hence, this
study employs High-Power Impulse Magnetron Sputtering (HiPIMS), which allows for
the deposition of high-quality GaN films at room temperature.
Firstly, the basic theory and conditions for waveguides are introduced. According
to Snell′s law, the core must be a high refractive index material to form a waveguide.
In this study, GaN based waveguide resonators are introduced. The coupling conditions,
quality factor, and group velocity dispersion of ring resonators are then explained. To
achieve critical coupling, it is necessary to have low-loss ring waveguides and enhance
coupling strength by minimizing the gap size. The coupling efficiency, mode numbers
of different waveguide widths, field distribution, and group velocity dispersion of GaN
and SiNx microring resonators with varying gap sizes are simulated using the FiniteDifference Time-Domain (FDTD) method and Finite Element Method (FEM). Due to
the higher refractive index of GaN, a smaller gap size is required to achieve critical
coupling. Additionally, a thinner insulating layer can be adopted due to the larger
refractive index of GaN.
Next, the fabrication process is introduced. GaN deposited using HiPIMS and
silicon nitride deposited using Low-Pressure Chemical Vapor Deposition (LPCVD) are
utilized as waveguide core materials. Since a gap size smaller than 300 nm is required
for critical coupling, an electron beam lithography system is used. Instead of the
traditional method of dividing the patterns into smaller writefields and sequentially
exposing them, this study employs the Fixed-Beam Moving Stage (FBMS) mode,
where the electron beam is fixed with moving stage for patterning, allowing for
exposure along the waveguide and avoiding stitching error issues that cause
transmission losses. After optimizing the exposure process, the transmission loss of
GaN waveguides is reduced from 57 dB to 32 dB with air cladding. With the deposition
of a silicon dioxide as the cladding layer, the transmission loss is further reduced to 16
dB, and the quality factor of GaN reaches 4×10^4
. This study is the first to achieve the
deposition of GaN on silicon dioxide and demonstrate a quality factor of 10^4
in a
microring resonator.
By utilizing HiPIMS for GaN deposition, this study provides a more versatile
deposition method for GaN devices onto different substrates. This technology enables
the integration of GaN with CMOS processes and holds great potential in realizing
photonic devices. | en_US |